Multiple-slide die-casting machines are known in the prior art, and they have at least two mold sections carried by shanks that are driven towards and away from each other. Molten metal is injected into the cavity formed between the two mold sections when the two mold sections are in a molding position and restrained together in a preloaded state. An example is described in applicant's U.S. Pat. No. 4,601,323, issued on Jul. 22, 1986. A machine for injection molding or die-casting according to that patent includes a main machine base with an injection unit mounted on the rear face and a mold guideway mounted on the front face. An aperture in the main machine base and a corresponding one in the base of the guideway provide for the nozzle of the injection unit to engage molds carried in the guideway. The reciprocating of the mold sections towards and away from one another is due to the action of a toggle assembly interconnecting mold carrying shanks with compression lever brackets mounted in the ends of the guideway, actuators located centrally of guideway ends and linked to the toggle assembly. Position adjusters are used for adjusting the location of the injection unit on the rear of the machine to position its nozzle relative to the molds.
It is important that the contact surfaces of the mold sections do not move, because they constitute the reference plane of the whole mold assembly. The contact plane is called the main parting line. However, in a preloaded state which is required to prevent the two mold sections from moving back while the pressurized melting metal is injected into the cavity between the mold sections, all the components of the clamping assemblies are stressed by the clamping force. The clamping force causes the table and the brackets which support the clamping assemblies to deflect because in a standard machine the said brackets are outrigged over the base. The pre-load force has to be higher than the reaction force induced by the injection pressure. Therefore, the deflection of the table and brackets caused by the clamping force is not to be ignored, and induces deformation of the mold guiding system which causes a misalignment of mold sections. Excessive wear of the slides and poor quality of molded product, such as flash formed along the parting line of the molded product, result from the base deflection and bracket deflection and mold mismatch.
There is a need for improvement of the structure of the machine to inhibit the deflection of the base in the preloaded state.
Study shows, nevertheless, the base deflection, bracket deflection and mold mismatch induced by the clamping force are not the only reason to produce the flash on the molded product. Hot chamber die-casting machines have traditionally been equipped with open loop control injection system. A key feature of the open loop control is that the pressure and flow rate of the hydraulic fluid supplied to the injection cylinder cannot be varied during the injection cycle. These parameters can be changed, but are fixed for any given injection cycle.
At the start of the cycle, hydraulic fluid fed to the injection cylinder causes the injection plunger to accelerate rapidly, then travel at approximately constant velocity to fill the cavity between the mold sections with melting metal. Once the cavity of the mold and runner system have been filled, all the moving components of the injection system come to a sudden stop. This results in a rapid increase in metal pressure within the cavity of the mold, called the “hammer effect” which often causes flash on the products. Although the degree of control over the injection process is somewhat limited with an open loop system, it is satisfactory for many applications.
For the past several years, closed loop control of the injection systems has been possible. Examples are described in U.S. Pat. No. 4,660,620, issued to Ozeki on Apr. 28, 1987, and U.S. Pat. No. 5,988,260 issued to Iwamoto et al. on Nov. 23, 1999.
Generally, the pressure and flow rate of the hydraulic fluid supplied to the injection cylinder in a closed loop control are changed during the injection cycle, and follow predetermined velocity and/or pressure profiles, and therefore the injection of the molten metal to the cavity of the mold is controlled in an optimum manner. However, the closed loop control of the injection system is currently used with large, conventional die-casting machines which have a relatively long injection time. That is because the system needs a certain minimum stroke to be able to react on and profile the injection. If a product (cast part) has to be molded which is smaller than one requiring the minimal stroke, it is typical to have to change a gooseneck of the injection system to install a smaller diameter sleeve and plunger which require a longer stroke to fill the same cavity of the mold. This is not an easy task. A small product can be produced in a very simple manner if the injection system of the machine can be switched from closed loop control to open loop control.
Therefore, there is a need for a multiple-slide die-casting machine which is adapted to change mold control mode easily from a closed loop control to an open loop control for different size products to be molded on the machine.